Electrostatic recording involves placing a substantial charge on very small discrete areas of a dielectric recording medium. Conventionally, the dielectric medium comprises, for example, a recording paper passed between a plurality of small, closely spaced styli and a backup plate or platen. Thereafter, a developer or toner is applied to the recording medium to render the charged areas visible. Subsequently, the recording medium with the toner applied is passed by a fixing station, usually a thermal element, which melts or otherwise causes the toner to be permanently fixed to the recording medium or paper.
To achieve gray scale recording it is necessary to place controlled charges on the dielectric material by the use of charging pulses of varying intensity. Then, depending upon the amount of charge on the paper the variable amount of toner will be deposited achieving a range of grayness between white and black. The number of levels of gray scale varies in different systems. The use of 16 gray scale levels is relatively well known.
In order to obtain resultant recordings having good quality, voltage ranges of the applied pulses between 300 and 600 volts are necessary to produce the requisite charge on the dielectric medium.
The development of modern electronic computers and their attendent ability to handle very large volumes of data in extremely short periods of time places heavy demands on output or printing apparatus. Thus, in order to operate a printer at a high data rate, it is necessary to produce the required high voltage charging pulses with short durations in order to produce a single gray scale picture element (pel) on the recording medium.
In order to accomplish such high printing speeds in an electrostatic gray scale recording system, extreme demands are placed on the pulse generating circuitry since the pulse producing process requires not only the fast switching, i.e., turn-on and turn-off of high-voltage levels, but also requires the generation of many different voltage levels within the writing range.